Re: [BangPypers] Evaluating a string against mapping in python.

[Noufal Ibrahim KV]

On Sun, Oct 26 2014, Okan bhan wrote:


[...]

 A simple implementation of this is:

 class SimpleMapping:
   def __init__(self, items):
 self._items = items

   def __getitems__(self, subitem):
 print('*' * 20)
 for item in self._items:
   if subitem in item:
 return True
 return False

This is broken in two ways
1. The magic method is __getitem__ (not __getitems__). I'm assuming it's
   a typo otherwise, the rest of your code will not work even as you've
   mentioned. 
2. A mapping object is similar to a dictionary. It should
   return the actual object if you try to access it. Not a boolean True
   or False. 

The mapping objects given are specifications of the locals and globals
in which you will evaluate your string. Consult help(eval) for more
information.

eval(source[, globals[, locals]]) - value

 Playing around against it for a bit.
  mapping = SimpleMapping(set(['aaa', 'bbb', 'ccc']))
  eval('ppp', {}, mapping)
 
 False
  eval('aa', {}, mapping)
 
 True
  eval('xxx and aaa', {}, mapping)
 
 False
  eval('xxx or aaa', {}, mapping)
 
 
 True

 Struggled in beginning but now they all seem obvious to me. Tricky is:

Look at my comment above. Your Mapping object is flawed and all these
responses are quite wrong.


  eval('5.6', {}, mapping)
 5.6

I'm not sure how this will work. In my case, I see this. 

 mapping = SimpleMapping(set(['aaa', 'bbb', 'ccc']))
 eval('5.6', {}, mapping)
Traceback (most recent call last):
  File stdin, line 1, in module
TypeError: locals must be a mapping
 


  eval('aa.a', {}, mapping)
 AttributeError: 'bool' object has no attribute 'a'


 My doubts are:
 a. Why it didn't run for numeric 5.6? Also why is dot separated '5.6' any
 different to 'aa.a'? I looked around on eval documentation and examples but
 couldn't find use of eval with a mapping.

5.6 is not an attribute access. 5.6 is is a floating point number. aa.a
is an attribute access. It will first return False for `aa` since there
is no such key and your code returns False and then try to access `a` in
the Boolean which, predictably, fails. 

 b. I see many blogs suggesting to refrain from using eval unless
 absolutely needed. Is this one use case we must use it? Do we have any
 better way to evaluate this?

I'm not sure what exactly you want to do. Can you explain again?

 c. If indeed, I have to evaluate a string containing dots, how to do
 in the above scenario?

The object whose attribute you're trying to access should be there in
the locals or globals and then should have an attribute with the given
name. It will work then.
[...]


-- 
Cordially,
Noufal
http://nibrahim.net.in
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[BangPypers] Evaluating a string against mapping in python.

[Okan bhan]

Hello everyone,

Hope you all celebrated festival of Diwali with your loved ones. Wishes
from my side to each one of you.

I'm facing issue in understanding python's 'eval' function's behaviour.
I've no previous deep knowledge of using 'eval' except that it is used to
evaluate a string just like on python prompt. While working on a feature
on pytest, I encountered usage of eval against a map.

For a very simple case, I wanted to check if a string exists within a list.
Very simple
   'foo' in list_foo.

But string 'foo' can contain expression (like 'foo and bar'). To check
this, I see they have used something like

 eval('foo and bar', {}, mapping).

A simple implementation of this is:

class SimpleMapping:
  def __init__(self, items):
self._items = items

  def __getitems__(self, subitem):
print('*' * 20)
for item in self._items:
  if subitem in item:
return True
return False

Playing around against it for a bit.
  mapping = SimpleMapping(set(['aaa', 'bbb', 'ccc']))
  eval('ppp', {}, mapping)

False
  eval('aa', {}, mapping)

True
  eval('xxx and aaa', {}, mapping)

False
  eval('xxx or aaa', {}, mapping)


True

Struggled in beginning but now they all seem obvious to me. Tricky is:

  eval('5.6', {}, mapping)
5.6
  eval('aa.a', {}, mapping)
AttributeError: 'bool' object has no attribute 'a'

My doubts are:
a. Why it didn't run for numeric 5.6? Also why is dot separated '5.6' any
different to 'aa.a'? I looked around on eval documentation and examples but
couldn't find use of eval with a mapping.

b. I see many blogs suggesting to refrain from using eval unless absolutely
needed. Is this one use case we must use it? Do we have any better way to
evaluate this?

c. If indeed, I have to evaluate a string containing dots, how to do in the
above scenario?

Would appreciate any pointers or more clarity on this.

Thanks  Regards
Alok
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